Screw feeder with multiple concentric flights

Abstract

A compound screw feeder for uniform volumetric flow rates of particulate solids, suitable for low, nonpulsating feed rates, comprises coaxial inner and outer screw flights. The flights both extend throughout the feeder section and produce a net discharge flow rate dependent on their respective feed directions and rates of feed. Various forms of the feeder may be employed in a loss of weight screw feeder system.

I claim: 
     
       1. A screw feeder for particulate solids having, in combination, a chamber having an end wall and wall portions extending downwardly and terminating laterally at said end wall,   a cylindrical feed tube external to the chamber and communicating therewith through an opening in said end wall, the feed tube extending from said end wall to a discharge opening,   feed screw means comprising coaxial helical inner and outer flights of differing outer diameters, each flight extending from within the chamber through said opening and the feed tube to the discharge opening, the outer flight being confined by the feed tube with a minimum nonbinding clearance, and   means to rotate the flights in directions whereby one flight feeds the solids in the direction from the chamber toward the discharge opening and the other flight tends to feed the solids in the opposite direction.   
     
     
       2. A screw feeder according to claim 1, in which the discharge opening is located in a wall of the feed tube. 
     
     
       3. A screw feeder according to claim 1, in which the flights are independently driven. 
     
     
       4. A screw feeder according to claim 1, in which the flights have differing individual material feed rates. 
     
     
       5. A screw feeder according to claim 4, in which the outer flight feeds the solids in the direction toward the discharge opening. 
     
     
       6. A screw feeder according to claim 1, in which the flights are of opposite pitch hand and are attached together. 
     
     
       7. A screw feeder according to claim 6, in which the feed screw means include a shaft and the inner and outer flights are fixed to the shaft. 
     
     
       8. A screw feeder according to claim 1, in which the outer flight comprises an open band of helical shape. 
     
     
       9. A screw feeder according to claim 8, in which the band is attached to the inner flight by struts to form an axially extending space between the flights. 
     
     
       10. A screw feeder according to claim 1, including a scale supporting the chamber and feeder and responsive to the loss of weight of material from the discharge opening.   
     
     
       11. A screw feeder according to claim 10, including a closed loop control adapted to sense the rate of change of the weight on said scale, and operatively connected to said means to rotate the flights in response to said rate. 
     
     
       12. A screw feeder for particulate solids having, in combination, a chamber having an end wall and wall portions extending downwardly to a trough, the lower portion of the trough conforming to a cylindrical sector, said trough terminating at said end wall,   a cylindrical feed tube external to the chamber, having substantially the same inside diameter as said sector and being coaxial therewith, the trough and one end of the feed tube communicating through an opening in the end wall, the feed tube extending from said end wall to a discharge opening,   feed screw means comprising coaxial helical inner and outer flights of differing outer diameters, each flight extending from within the trough through said opening and the feed tube to the discharge opening, the outer flight being confined by the feed tube with a minimum nonbinding clearance, and   means to rotate the flights in directions whereby one flight feeds the solids in the direction from the chamber toward the discharge opening and the other flight tends to feed the solids in the opposite direction.   
     
     
       13. A screw feeder according to claim 12, in which said wall portions slope tangentially to said cylindrical sector to form a V-shaped trough. 
     
     
       14. A screw feeder according to claim 12, in which the chamber comprises a U-shaped trough and a wall sloping to the sides of the trough to form an elongate aperture over the feed screw.